This invention relates to a method and an apparatus for the continuous preparation of aqueous solution of slaked lime. Although not limitative, such aqueous solution is broadly used for adjusting pH values in the city water main lines so as to avoid the formation of iron rust. As an alternative use of such aqueous solution, it is used for adding calcium ions to artificially regenerated fresh water from sea water.
As the pH-modifier for city water mains, caustic potash has been broadly used. Recently, however, in place of the potash, slaked lime is increasingly utilized, because the latter is capable of forming a kind of stable protective layer on the inside wall surfaces of city water iron pipe lines.
However, there is a grave difficulty in the use of slaked lime which is rather difficult to dissolve in water. Inadvantageous local deposits or the like are liable to form on the inside wall surfaces of the city main pipe lines. Thus, for using slaked slime for this purpose, a large capacity of reservation tank is necessary to provide a chance of supplying clear aqueous solution of slaked lime.
In last decade, continuously operable apparatus for the production of aqueous slaked lime solution have been proposed and found their gradually increased usage.
As a generalized representative apparatus used for the above purpose, it comprises a tank or vessel, the interior space thereof being, however, separated at a lower level into two by a separating wire net arranged to place thereon the material slaked lime forming a relatively thick layer. The water for the wanted aqueous slaked lime solution is constantly supplied preferably at a predetermined constant rate to the place in close proximity of the tank bottom, while the thus formed saturated product solution is constantly taken out from the preparation tank at the top thereof.
It has been found that for several initial hours of the lime-dissolving operation, such plants work substantially satisfactorily. With lapse of further time, however, the concentration of the product solution will decrease gradually, on account of such phenomenon that a number of relatively large liquid passages are formed through the solids material layer and the sizes of these passages will become still larger with further continued time of the dissolving operation.
It will thus be seen that the water may pass through these gradually enlarged liquid passages and without intimate contact with the solids material.
Another difficulty is caused by clogging of the passage openings of the solids-supporting wire net as appearing in an extended use of the plant for at least several months. More specifically, a small amount of sand or the like insoluble particles contained in the material solids will gradually accumulate at the openings of the wire net. On the other hand, the city water may include unavoidably certain amount of iron rust or the like insoluble solid particles which give rise also to net clogging. Further, a slight amount of free acid(s) may be included in the material water which may react with Ca(OH).sub.2 formed in the solution to CaCO.sub.3, thereby providing liably clogging of the wire net openings.
The invention has its object to provide an efficiently operable apparatus adapted for the continuous formation of normally saturated liquid solution of slaked lime, without inviting above mentioned several conventional drawbacks, especially without use of the solids-supporting wire net means, and in a highly stable manner for an extended period of time.
The invention relates further to a process for the continuous preparation of the above kind.